Many engineering applications require an insulating layer to be coated on a metallic body to provide certain functionality, for example, protection against chemical, mechanical, thermal or electrical influences. Most insulating materials in use today are ceramics. However, often due to the difference in the coefficients of thermal expansion between the ceramic coating and the underlying metallic body, high mechanical stresses are generated in the ceramic coating under thermal loads. These stresses readily lead to cracking and/or delamination of the coating. Therefore, a insulating material which has a coefficient of thermal expansion similar to that of the metallic material to be coated is desirable.
Metals generally have a coefficient of thermal expansion which is greater than 10 μm/m/K, therefore only a few oxides may be used for coating purposes. Stabilized zirconium oxide, for example, with coefficient of thermal expansion in the region of 11 μm/m/K is used in turbo-machinery components as a thermal barrier coating and in electrical devices such as solid oxide fuel cells as an ionic conductor (electrolyte) at high temperature. However, the resistance of zirconium oxide to attack from metallic or oxide melts is lower than that of a number of other materials [Ref U.S. Pat. Nos. 6,723,442; 6,764,771, incorporated herein by reference in their entireties]. Zirconium oxide also loses its electrical insulating properties at high temperature where ionic conduction predominates.
For application of the coating by thermal spray process, the insulating material additionally needs to have suitable properties to withstand the process conditions and form coatings with desired functionality. For example, MgO has a high melting point, sufficient resistance to melts, high electrical insulation and a coefficient of thermal expansion of 13.5 μm/m/K. This implies that MgO is also a suitable coating material for metals. However, MgO is not a suitable material for use in a thermal spraying process, since MgO decomposes at high temperatures which occur in such processes, and the decomposition products are volatile.
Ceramics which are produced from a mixture of MgO and Al2O3 have good properties for use in combination with various metals. Sintered ceramics produced from MgO and Al2O3 are commercially available. They have the advantages of being highly resistant to chemical, thermal and mechanical attacks and of having a coefficient of thermal expansion which lies in the region of 11 μm/m/K. However, ceramics of this type have only limited suitability as coating material, since in practice they are not suitable for coating by way of a thermal spraying method. In these ceramics too, the MgO of the ceramic evaporates at the high temperatures which occur during thermal spraying.
U.S. Pat. No. 6,723,442 describes a material based on the combination of MgAl2O4 spinel and MgO, a method of its production, a coating (or layer) produced from the material applied on a metallic body and the use of such coated metallic body as a component in a high temperature fuel cell. The material is described as comprising grains of MgO which are embedded in a matrix of the spinel MgAl2O4.
U.S. Pat. No. 6,764,771 describes a (metallic) turbine blade coated with a thermal barrier coating which is based on an admixture of a spinel material selected from a group of CoMg2O4, CoFe2O4, CoCr2O4, CoTi2O4, CoAl2O4, NiMg2O4, NiTi2O4, TiMg2O4, TiFe2O4, TiCr2O4, and TiAl2O4, and oxide material selected from a group of MgO, HfO2, CoO, NiO, and Cr2O3 and/or combinations thereof.
US patent application publication US2011/0033779 (Feb. 10, 2011), incorporated herein by reference in its entirety, describes a several material compositions and combinations for use as insulation in solid oxide fuel cell (SOFC) systems.
There remains a need for materials and methods to deposit thermal spray coatings comprising, e.g., magnesium oxide and a spinel, preferably MgAl2O4.